1. Field of the Invention
The present invention relates generally to methods and apparatus for recovering volatile liquids from particulate materials. More particularly, the present invention relates to processes and equipment for recovering hydrocarbons from coal agglomerates where the hydrocarbons have been used as a bridging liquid in agglomerate formation.
Size enlargement of particles contained in a slurry may be performed for a variety of purposes, including filtration, dewatering, settling, and the like. In one type of size enlargement process, a bridging liquid is combined in a liquid suspension of fine particles. By agitating the suspension, the bridging liquid acts to selectively wet the surfaces of the particles causing the desired agglomeration. Of particular interest to the present invention, are coal agglomeration processes where coal fines suspended in a water slurry are combined with hydrocarbon liquid and subjected to agitation. The resulting size enlargement of the coal greatly facilitates handling characteristics and significantly reduces wastage.
Coal processing frequently involves washing to remove various mineral and inorganic substances which can cause "ash" on subsequent burning. The formation and loss of fine coal particles is an unfortunate side effect of such washing processes. Such loss of coal is not only wasteful, it also causes secondary water pollution.
The coal fines resulting from washing are usually contained in a tailing stream from the primary washing process. An effective approach for removing the coal fines is known as "selective agglomeration" where coal is extracted from the aqueous suspension medium using hydrocarbons as a bridging liquid, as described above.
A particularly effective coal fines agglomeration process has been developed by the Arcanum Corporation, Ann Arbor, MI. In a two-step process, heptane is first added to a coal-water slurry to initiate aggregation and cause the formation of "micro-agglomerates." In the second step, a heptane-asphalt mixture is combined with the micro-agglomerates to cause further size enlargement, typically to a final size in the range from about 3 to 8 millimeters. With this size, the water and dispersed minerals may easily be removed from the agglomerates.
Heptane, however, is a volatile, flammable, and expensive material which must be removed from the agglomerates for both safety and economic reasons. After removal of the heptane, the coal agglomerates will remain bound by the presence of asphalt which was introduced in the second step. The removal of heptane from the agglomerates may be achieved by applying heat to evaporate the heptane which can then be collected and recovered. The use of steam in a batch process for the recovery of heptane has been demonstrated, however, under batch conditions, direct steam heating can cause sticking and lumping in the stationary bed of solids, requiring increased processing time and making the final product more difficult to transport and manipulate. Direct steam heating can also cause excessive thermal gradients in agglomerate particles which can result in vapor evolution at rates sufficient to rupture said particles. Furthermore, a continuous processing system would have well known economic advantages over a batch system.
For these reasons, it would be desirable to provide methods and apparatus for recovering heptane and other hydrocarbons from coal agglomerates where hydrocarbons have been used as a bridging liquid. It would be further desirable if such methods and apparatus utilized steam and/or other sources of heat, but minimized the shortcomings of the aforementioned batch process. Such methods and apparatus should provide a relatively non-sticky, free flowing product to facilitate subsequent handling and combustion, while recovery of the heptane and light hydrocarbons should be performed under conditions which minimize the likelihood of fire and explosion. Finally, substantial recovery of heptane and/or other light hydrocarbons should be achieved, preferably being greater than 98%, more preferably being greater than 99%.
2. Description of the Background Art
Size enlargement (agglomeration) of fine coal particles using hydrocarbon oils as a bridging liquid is taught in U.S. Pat. Nos. 4,033,729 and 4,874,393. A method for deashing coal which has been agglomerated with a hydrocarbon liquid is described in U.S. Pat. No. 4,396,396, where the hydrocarbon liquid may be recovered by vacuum stripping, with or without superheated steam. Other processes for washing and agglomerating coal particles are described in U.S. Pat. Nos. 4,559,060; 4,484,928; 4,447,245; 4,255,155; 4,209,301; 4,173,530; and 4,081,571. U.S. Pat. No. 4,102,968 teaches a method for agglomerating sulfur particles with molten sulfur, and U.S. Pat. No. 4,213,779, teaches a method for agglomerating iron wastes with organic bridging liquids. The disclosures of each of these patents are incorporated herein by reference.